1. Field of the Invention
This invention relates to a focus adjusting device for a camera, and more particularly to a focus adjusting device in which an electrostriction device, such as a laminate piezoelectric device, is used and a deformation of the electrostriction device is increased or accumulated for moving an imaging unit, such as a lens system, film, or imaging device.
2. Discussion of the Background
A focus adjusting operation for an auto focus (AF) for a camera has been performed as follows. In a camera using a photographic film, a focus adjusting is typically performed by either (1) moving a part or whole of a lens group, which constitutes an optical system, along the optical axis of the lens group, or (2) by moving a position of a film in the direction of the optical axis of the lens group. In a digital camera, a focus adjusting is also typically performed by either (1) moving a part or whole of a lens group, which constitutes an optical system, along the optical axis of the lens group, or (2) by moving an imaging device, for example a charge coupled device (CCD) or CMOS sensor, in the direction of the optical axis of the lens group.
In both of the above-described film camera and digital camera, a motor, such as a DC motor, a stepping motor, an ultrasonic motor, etc., is generally used as a driving source for the focus adjusting.
However, the following problems arise by using a motor as a driving source. First, the space required for the motor, and a gear reduction block that is necessary for the motor, prevent providing a compact camera. Second, the motor typically has a rising-time characteristic and a multiple-pulse driving, and as a result a relatively long time interval is needed to move a lens, a film, or an imaging device, to a predetermined distance. Third, a driver circuit only for driving the motor is necessary, and thereby costs are increased.
In order to solve the above-described problems, for example, a method of using a piezoelectric device for focusing is described in "LENS DEVICE" of Japanese Laid-Open Patent Publication No. 8-94906/1996.
In the above lens device, focusing is performed by using a piezoelectric actuator to move a lens group along an optical axis of the lens group. Specifically, a pin of a holding frame holding a lens group is made to pierce through a cam groove of a focus cam ring and engage with a cam groove of a zoom cam ring. The focus cam ring is driven by a driving section which is disposed in an optical axis direction and which connects to a piezoelectric actuator, and friction is produced between the driving section and the focus cam ring by deformation of a piezoelectric member of the piezoelectric actuator. When a voltage with driving pulses is applied to the piezoelectric actuator, the focus cam ring is moved in the optical axis direction, and thereby the focusing is performed.
More specifically, a basic method of moving the focus cam ring forward and backward in JP No. 8-94906/1996 is now explained referring to FIGS. 21 and 22. To make the explanation simple and clear, an exemplary structure of a driving device is illustrated simply in FIG. 21. The driving device includes a piezoelectric actuator 30 with one end face fixed to a flange 33, a rod 31 secured to another end face of the piezoelectric actuator 30, the rod 31 being supported for moving in the direction of an optical axis of a lens group, and a ring 32 engaged with the rod 31 (FIG. 21(A)). When a voltage with a slow rising-time pulse is applied to the piezoelectric actuator 30, the piezoelectric actuator 30 expands slowly, and then the rod 31 and the ring 32 move forward (FIG. 21(B)). When a voltage with a steep falling-time pulse is applied to the piezoelectric actuator 30, the piezoelectric actuator 30 contracts rapidly, and then the rod 31 moves backward. However, in this contracting operation the ring 32 is not contracted from its expanded position because the inertia force of the ring 32 exceeds the friction force produced between the ring 32 and the rod 31 (FIG. 21(C)). Thus, when the aforementioned driving pulses having the slow rising-time and the steep falling-time are repeatedly applied to the piezoelectric actuator 30 (FIG. 22(A)), the ring 32 moves forward successively with each driving pulse. On the other hand, when driving pulses having a steep rising-time and a slow falling-time are repeatedly applied to the piezoelectric actuator 30 (FIG. 22(B)), the ring 32 moves backward.
Thus, in this background art by moving the focus cam ring 32 directly in the optical axis direction by the deformation of the piezoelectric member 30, the construction of the driving device of the lens device can be simpler than in a driving device using a motor. As a result, a number of parts, a size, and a weight of the driving device can be reduced.
However, the above-described lens device has difficulties in providing an accurate focusing because constant friction between the driving section and the focus cam ring 32 is always required to move the focus cam ring 32 engaged with the lens group forward or backward continuously while changing the rising-time and falling-time of the driving pulses of the voltage applied to the piezoelectric actuator 30. For example, a quality of component parts, an inclination of the camera, and environmental conditions, such as a temperature, influence this friction. Also, the lens device needs to drive with multiple-pulses to move the piezoelectric actuator 30 to a predetermined distance, and this still requires significant time.